The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reverse osmosis systems typically use one or more membrane housings that have one or more membranes therein that are used to extract an essentially pure fluid from a solution. The desalination reverse osmosis membranes receive feed fluid from brackish or sea water and extract fresh water therefrom. Fresh water is extracted or separated when the pressure of the feed fluid exceeds the osmotic pressure of the fluid which allows permeate or product fluid to cross the semi-permeable reverse osmosis membrane. The fluid that is left on the input side to the membrane becomes higher in salt concentration because fresh water that travels through the membrane does not include the salt. The water that passes through the membrane is referred to as permeate. The pressure required to produce fresh water is proportional to the concentration of the total dissolved solids (TDS) in the feed solution within the reverse osmosis housing. For typical ocean water, the concentration is about 35,000 parts per million (ppm) and the corresponding osmotic pressure is about 450 pounds per square inch (psi) (3,102 kPa). For 70,000 ppm feed fluid, the osmotic pressure approximately doubles to 900 psi (about 6,205 kPa). A typical seawater reverse osmosis system uses a series of membranes that recover up to about 45% of the fresh water and generate about 55% concentrate brine from the original volume of seawater. The net driving pressure (NDP) equals the feed pressure minus the osmotic pressure minus the permeate pressure. The net driving pressure is the pressure energy available to drive pure fluid across the membrane.
Referring now to FIG. 1A, a reverse osmosis system 10 according to the prior art includes a membrane array 12 that generates a permeate stream through permeate pipe 14 and a brine stream through a brine pipe 16 from a feed stream in a feed pipe 18. The feed stream originates from a source 19 typically includes brackish or sea water. A feed pump 20 coupled to a motor 22 pressurizes the feed stream to a required pressure, and the feed stream enters the membrane array 12 at the required pressure.
The membrane array 12 includes a membrane housing or pressure vessel 24 and a membrane 26. The portion of the feed stream that flows through the membrane 26 before exiting the membrane array 12 forms the permeate stream that exits through the permeate pipe 14. The portion of the feed stream that does not flow through the membrane 26 before exiting the membrane array 12 forms the brine stream that exits in the brine pipe 16.
The permeate stream in the permeate pipe 14 is a purified fluid flow at a low pressure that collects in a tank 28 or is piped to a desired location. The brine stream is a higher pressure stream that contains dissolved materials blocked by the membrane 26. The pressure of the brine stream is only slightly lower than the feed stream. A control valve 30 may be used to regulate the flow through and pressure in the membrane array 12. The brine stream may flow through the control valve 30 and into a drain or tank 32.
Referring now to FIG. 1B, the membrane 26 of FIG. 1A is typically formed of a plurality of elements 40. The elements 40 are typically formed in a cylindrical shape by rolling a plurality of sheets and spacers together. In this example a first sheet 42 and a second sheet 44 are glued together on three sides with the fourth side being in glued communication with the central collection tube 46 communicating permeate to a desired location as indicated by arrow 48. Brine which may also be referred to as reject 50 does not enter the collection tube 46. The sheets and the spacers 52 are glued between the membrane sheets 42 and 44 to allow the sheet 44 to stay slightly apart and allow permeate to flow to the collection tube 46. A second spacer sheet 54 is used to keep the membrane sheets slightly apart and allow the axial flow through the element and allow brine or reject 50 to flow therethrough.
Referring now to FIG. 1C, a membrane channel 56 is used to deliver the feed fluid. The membrane sheets 42 and 44 are illustrated. The membrane channel 56 has an inlet 56A and an outlet 56B through which the feed fluid progresses. As the feed fluid progresses through the membrane channel 56, the concentration of dissolved solid increases. This is represented by the permeate 58. Permeate production is much higher at the inlet 56A of the membrane channel 56 and decreases over the length of the membrane channel 56 toward the outlet 56B. Along the length of the membrane channel 56 the total dissolved solids (TDS) increases and thus the higher osmotic pressure and a reduction in feed pressure is present over the length of the membrane channel 56. A reduction in the net driving pressure (NDP) is also present as the permeate is extracted down the length of the permeate channel 56.
Referring now to FIG. 1D, a chart illustrating the relationships of various membrane parameters for a reverse osmosis system with about forty-five percent recovery in the handling of sea water is set forth. In this example the feed pressure is about 860 psi (5929 kPa) and loses about 10 psi (68.95 kPa) over the channel length. The osmotic pressure is about 450 psi (3103 kPa) and rises to about 820 psi (5654 kPa) due to the increasing total dissolved solids of the feed. The feed total dissolved solids (TDS) starts at about 35,000 ppm and raises to 63,000 ppm at the end of the membrane channel 56 illustrated in FIG. 1C. The net driving pressure (NDP) starts about 500 psi (3447 kPa) and decreases to about 50 psi (345 kPa). The permeate flow rate decreases to a negligible amount at the end of the membrane channel 56.
Referring now to FIG. 1E, an inlet pipe 60 fluidically communicates fluid into the pressure vessel 24. A flow distributor 62 distributes fluid to the reverse osmosis elements 40A-40E in-series rather than around the elements 40A-40E. The flow distributor 62 spreads the fluid flow radially across the surface of element 40A. The seal 64 allows fluid from the flow distributor 62 to not circumvent the first element 40A. The flow continues through the elements 40A-40E sequentially. Permeate exit collection tubes 46A, 46B, 46C, 46D and 46E receive the permeate from each respective element 40A-40E. Connectors 66A-66D join successive permeate exit collection tubes 46A-46E. An anti-telescoping device 68 may be used to maintain the position of the elements 40A-40E relative to the flow distributor 62. In most applications between three and eight elements are used. Five of which are used in this example. A brine exit pipe 70 is used to emit the brine from the pressure vessel 24. Permeate exit collection tube 46 flows in a direction indicated by the arrow 48.
As the feed progresses from element to element, the amount of total dissolved solids (TDS) increases until the brine exits the brine exit pipe 70. The osmotic pressure is mostly determined by the concentration of the total dissolved solids. Each succeeding element experiences a higher concentration and thus higher osmotic pressure and lower Net Driving Pressure than the preceding element. Consequently, each successful element has lower permeate production than the preceding element. A minimum Net Driving Pressure for sea water in an RO system is about 100 psi (689.5 kPa). An initial feed pressure must be substantially higher than the initial osmotic pressure to ensure sufficient Net Driving Pressure available toward the end of the array. A typical pressure may be about 800 psi (5516 kPa) while the osmotic pressure is about 450 psi (3103 kPa) which yields a Net Driving Pressure of 350 psi (2413 kPa) for the first element. At the end of the array the osmotic pressure may be 700 psi (4826 kPa) which reduces the Net Driving Pressure to 100 psi (689.5 kPa). A high initial Net Driving Pressure is wasteful because the pressure is much higher than needed for an optimal rate of permeate production. In an ideal situation, the feed pressure would steadily increase to compensate for the increasing osmotic pressure resulting in a constant net driving pressure throughout the array.
A valve 72 is set to increase permeate pressure to reduce flux in the element to an acceptable value. The higher permeate pressure reduces differential pressure and thus is reduced and fouling is reduced. However, this causes other membranes to have reduced NDP and thus low productivity. This may result in a final membrane producing little or no permeate.
Another issue with reverse osmosis systems is polarization. Polarization is the formation of a stagnant boundary layer adjacent to the membrane surface where the concentration of salinity and foulant becomes very high. Polarization occurs when the flow velocity through the membrane elements is reduced to a certain value. Polarization typically becomes severe when flow velocity drops to below fifty percent relative to the inlet flow velocity of the first element. The typical amount of permeate that can be recovered is about fifty percent or lower and may have a typical range between thirty-eight and forty-five percent.
Referring now to FIG. 2A, one way in which to achieve higher permeate recovery is employing a first set of pressure vessels 210A, 210B which feed a second set of pressure vessels 210C. In this example, two pressure vessels are illustrated in a first stage 212 and a single pressure vessel is illustrated in a second stage 214. This type of configuration is referred to as a 2:1 array. Feed fluid enters a feed manifold 220 which is distributed between the pressure vessels 210A and 210B. The brine exits the pressure vessels 210A and 210B through a brine manifold 224 to pressure vessel 210C in the second stage 214. Permeate exits the pressure vessels 210A and 210B through a permeate manifold 228. The permeate manifold 228 is also in communication with the permeate generated in the pressure vessel 210C. The higher concentrated brine is removed from the pressure vessel 210C through a brine pipe 230. Of course, other types of array configurations are known such as a 3:2 and 4:3. For three-stage systems 6:4:2 configurations have been used. Two-stage systems have permeate recovery of about fifty percent to seventy-five percent. Three stage systems may also recover up to about eighty-five percent of permeate.
A valve 232 is set to increase permeate pressure sufficiently to reduce the flux in the first element to an acceptable level. Elements in the second stage operate with normal permeate pressure and thus maximum NDP is available.
A second example of a two-stage system is illustrated in FIG. 2B. In this example, a boost pump 240 is used between the two stages. That is, the boost pump 240 is in communication with the brine manifold 224 and boosts the pressure in the brine manifold 224 to a desirable pressure to compensate the losses in the Net Driving Pressure that occur within the pressure vessels 210A and 2108 of the first stage 212. Energy recovery devices such as turbochargers are known to be used in reverse osmosis systems to recover the hydraulic energy in a brine stream that exit the last stage and boosts the pressure of another stream such as the feed stream.